Distortion periodically occurs in a rubber portion of a tire during running, a portion of its energy is converted into heat and heat is generated. Especially when a vehicle runs in a state where an air pressure is reduced, the distortions of various portions are increased and heat generation is also increased. If a rubber temperature of the tire exceeds a certain value, destruction of the rubber starts. Therefore, in order to enhance the durability of the tire, it is effective to lower a temperature of the tire during running.
In recent years, to enhance convenience and safety and to widen a vehicle interior space, run flat tires are popular. A side-reinforced run flat tire that includes a side-reinforcing rubber layer having a substantially crescentic cross section inside of each of sidewalls is well known (see the following patent literature 1 for example). According to the run flat tire, even if the tire blows, the side-reinforcing rubber layer supports a tire load instead of the air pressure and deflection of the sidewall is limited. This retards the heat generation of the tire. Therefore, even if the run flat tire blows, it is possible to continuously run about 50 to 100 km at a speed of 60 to 80 km/h (such running is referred to also as “run flat running” hereafter).
However, at the time of run flat running where an air pressure is reduced, even the run flat tire generates heat in the side-reinforcing rubber layer in proportion to a running distance, and if the running distance exceeds a limit, the side-reinforcing rubber layer is destroyed due to heat generation.    [Patent Document 1] Japanese Patent Application Publication No. 2006-182318